Heretofore, it has been common that thin-film type electroluminescent devices are made of inorganic material which is obtained by doping a semiconductor of a Group II-VI compound such as ZnS, CaS or SrS with Mn or a rare earth element (such as Eu, Ce, Tb or Sm) as the luminous center. However, the electroluminescent devices prepared from such inorganic materials, have problems such that 1) alternate current driving is required (about 1 KHz), 2) the driving voltage is high (about 200 V), 3) it is difficult to obtain full coloring, and 4) the cost for peripheral driving circuits is high.
In order to overcome such problems, there have been activities, in recent years, to develop electroluminescent devices using organic materials. As the materials for the luminescent layer, in addition to anthracene and pyrene which were already known, cyanine dyes (J. Chem. Soc., Chem. Commun., 557, 1985), pyrazoline (Mol. Cryst. Liq. Cryst., 135, 355, (1986)), perylene (Jpn. J. Appl. Phys., 25, L773, (1986)) or coumarin compounds and tetraphenylbutadiene (Japanese Unexamined Patent Publication No. 51781/1982), have been reported. Further, it has been proposed to optimize the type of electrodes or to provide a hole injection transport layer and a luminescent layer composed of an organic phosphor, for the purpose of improving the injection efficiency of a carrier from the electrodes in order to increase the luminous efficiency (Japanese Unexamined Patent Publications No. 51781/1982, No. 194393/1984 and No. 295695/1988).
Further, for the purpose of improving the luminous efficiency of the device and modifying the luminescent color, it has been proposed to dope a fluorescent dye for laser, such as coumarin, using an aluminum complex of 8-hydroxyquinoline as host material (J. Appl. Phys., Vol. 65, p. 3610, 1989).
With organic electroluminescent devices heretofore disclosed, electroluminescence is brought about by recombination of injected holes and electrons. However, usually, injection of a carrier has to be conducted by overcoming an injection barrier at the interface between the anode and the organic hole injection transport layer in the case of holes, or by overcoming a barrier at the interface between the cathode and the luminescent layer in the case of electrons. Therefore, a high electric field is required for the injection. Accordingly, the driving voltage for the device is required to be high, and the luminous performance, particularly the luminous efficiency, tends to be inadequate. Further, instability of the operation due to instability of the interface is observed. Therefore, further improvements have been desired.